1. Field of Invention
The present invention relates to a method of shaping a specimen block with cutting and grinding operations. More particularly, the present invention relates to a method of shaping a specimen block with cutting and grinding operations so that the specimen block is ready for observation in a transmission electron microscope (TEM).
2. Description of Related Art
In analyzing the failure of VLSI device, cross-sectional analysis of a particular target point is an effective technique. A scanning electron microscope (SEM) is a convenient tool for observing the cross-section of a target point. However, because the SEM has a poor resolution for high-density materials, the SEM is now being replaced by a transmission electron microscope (TEM) as the means of performing failure analysis ever since the semiconductor manufacturing industry has shifted towards producing more ULSI devices. As the popularity of the TEM increase, convenient methods for preparing a specimen block of suitable thickness for observation have become an important issue. A specimen must have a thickness smaller than 0.25 .mu.m before it is transparent enough for TEM observation. Therefore, a focus on ion beam (FIB) method has been developed for removing additional material from the cutout block so that a very thin section is obtained. FIG. 1 shows the ultimate shape of a specimen for TEM observation after a series of preparatory steps. The method of preparation includes first cutting out a specimen block 100 from a marked portion of the wafer. The specimen block 100 has a thickness d of about 30 .mu.m. Next, the specimen block 100 is further ground in a top-down direction 102 using a focused ion beam so that the marked portion 104 of the specimen has a very thin section. Now, the marked section 104 can be observed using a TEM from a direction 106.
FIGS. 2A through 2G are a series of views showing a conventional method of preparing a specimen block, wherein the cross-mark contains a portion of the defective device that needs to be observed by a TEM. First, as shown in FIG. 2A, a laser is used to form a mark 202 locating the position of a failed device on a specimen block 200. The mark 202 is the so-called target point 202. Next, the specimen block 200 is ground, starting from one end 204 of the specimen block 200. The grinding only stops when the surface has come within 30 .mu.m of the target point 202 as shown in FIG. 2B. Due the lack of any labels for assessing distance from the target point, the grinding machine has to be stopped frequently, especially near the end of the grinding operation. Therefore, if one is not careful enough, over-grinding can easily occur leading to destruction of the target point 202.
Next, as shown in FIG. 2C, the final specimen block 200 as shown in FIG. 2B is placed on a jig 206. The jig 206 is made by attaching the backs of two unwanted wafer chips 206a and 206b together. The specimen block 200 is placed on the exposed surface of the wafer chip 206a. The location of the target point 202 must not be too far away from the end 204 as shown in FIGS. 2B and 2D so as to avoid the possibility of breaking the target point 202. Next, as show in FIG. 2E, the specimen block 200 and the wafer chip 206a are simultaneously ground in a direction 207 so that a portion of the unwanted specimen block 200 is removed. The wafer chip 206a of the jig is a sacrificial material. Finally, a specimen block 200 having a thickness of about 30 .mu.m is obtained. Thereafter, the specimen block 200 is placed on a copper grid 208 as shown in FIG. 2F, and then the copper grid 208 is placed on a base block 210 as shown in FIG. 2G. The specimen block 200 is further ground by a focused ion beam 212 so that the target point 202 becomes a very thin section similar to the one shown in FIG. 1. Now, the specimen block 200 is ready for observation by a TEM from direction 214.
In the above procedure of cutting and grinding to obtain a specimen block, over-polishing of the specimen can easily happen, resulting in a damaged target point. This is because polishing thickness is difficult to control. On the other hand, if the specimen block is only polished a little to avoid damaging the target point, the specimen block will be too thick for the focused ion beam to operate. Therefore, the processing time for the focused ion beam will be considerable.
In light of the foregoing, there is a need to improve the method of preparing specimen blocks.